Plasma arc torches are known in the prior art, and comprise a device which can efficiently convert electrical energy into heat energy. The torch generates a plasma flame using a relatively small gas flow. The heat from the plasma flame has advantages over the heat from a combustion flame, including high flame temperature, variable controlled atmospheres which are compatible with many chemical and metallurgical processes, and high thermal efficiencies. Normally, about 9 kilograms (at 20 pounds pressure) of air are required to release through the combustion flame the 11,000 kilocalories (44,000 BTUs) in one kilogram of fuel oil or one kilogram of natural gas. The torch plasma flame on the other hand requires less than 0.1 kilogram of air to release comparable heat levels. The high flame temperature, low gas requirement and high thermal efficiencies make the plasma arc torch ideally suited for various applications in the rapidly growing technologies such as aerospace, nuclear, and energy fields, and in the more conventional technologies such as waste disposal, steelmaking and refining, ozone generation, and the like.
The plasma arc generators or torches are commonly of two types. The first type utilizes the more conventional non-transferred arc mode of operation. The plasma generator or torch for operation in the non-transferred arc mode comprises a rear electrode, a front electrode, and a gas vortex generator which is coaxially aligned between the two electrodes. The entire assembly is, of course, contained within a suitable housing which may be water-cooled and has the necessary ancillary power components necessary for generating an electrical arc, which extends from the rear electrode, through the gas vortex generator, and to an attachment point on the front electrode. The second mode of operation is the so-called transferred arc type. In the transferred arc type of plasma generator, a collimating nozzle is mounted in coaxial alignment with the rear electrode and vortex generator. In this type of operation, the electrical arc attaches between the rear electrode and an external workpiece which is being worked upon, after passing through the collimating nozzle. Transferred arc generators are described, for example, in Baird, U.S. Pat. No. 3,194,941; and in Camacho, U.S. Pat. No. 3,818,174. The two modes of operation have advantages in their select areas of application.
In copending application Ser. No. 460,062 filed Jan. 21, 1983, various improvements are described in plasma arc torches, including a torch capable of being operated in the transferred arc mode as well as in the non-transferred arc mode, note the specification at pages 26-29, and FIGS. 71-77. The application also discloses at FIG. 72 an improved front electrode which can be assembled with the general assembly of the described torch. This torch has been referred to at times as a convertible torch. The present application is concerned with a convertible torch of the type defined in application Ser. No. 460,062.